U.S. patent application number 15/078472 was filed with the patent office on 2016-09-29 for energy storage device.
The applicant listed for this patent is GS Yuasa International Ltd.. Invention is credited to Kazuki KAWAGUCHI, Masaki MASUDA, Takeshi SASAKI, Taro YAMAFUKU.
Application Number | 20160285074 15/078472 |
Document ID | / |
Family ID | 55588117 |
Filed Date | 2016-09-29 |
United States Patent
Application |
20160285074 |
Kind Code |
A1 |
YAMAFUKU; Taro ; et
al. |
September 29, 2016 |
ENERGY STORAGE DEVICE
Abstract
An energy storage device includes an electrode having an
electrode substrate; an active material layer which is disposed to
cover a surface of the electrode substrate and which contains
active material particles; and an intermediate layer which is
disposed between the electrode substrate and the active material
layer and which contains a binder, wherein the active material
particles of the active material layer enter the intermediate
layer, and are in contact with the electrode substrate and the
intermediate layer.
Inventors: |
YAMAFUKU; Taro; (Kyoto,
JP) ; MASUDA; Masaki; (Kyoto, JP) ; KAWAGUCHI;
Kazuki; (Kyoto, JP) ; SASAKI; Takeshi; (Kyoto,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GS Yuasa International Ltd. |
Kyoto-shi |
|
JP |
|
|
Family ID: |
55588117 |
Appl. No.: |
15/078472 |
Filed: |
March 23, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/667 20130101;
H01M 4/668 20130101; H01M 4/622 20130101; H01M 2004/021 20130101;
H01G 11/46 20130101; H01G 11/30 20130101; H01G 11/32 20130101; Y02E
60/10 20130101; H01M 4/1391 20130101; H01G 11/60 20130101; H01M
4/131 20130101; H01M 2004/028 20130101; H01M 4/505 20130101; H01M
4/625 20130101; H01M 4/366 20130101; H01M 4/525 20130101; H01M
10/0525 20130101 |
International
Class: |
H01M 4/131 20060101
H01M004/131; H01M 10/0525 20060101 H01M010/0525; H01G 11/32
20060101 H01G011/32; H01G 11/60 20060101 H01G011/60; H01G 11/30
20060101 H01G011/30; H01G 11/46 20060101 H01G011/46 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 26, 2015 |
JP |
2015-064186 |
Claims
1. An energy storage device comprising: an electrode including an
electrode substrate; an active material layer which is disposed to
cover a surface of the electrode substrate and which contains
active material particles; and an intermediate layer which is
disposed between the electrode substrate and the active material
layer and which contains a binder, wherein the active material
particles of the active material layer enter the intermediate
layer, and are in contact with the electrode substrate and the
intermediate layer.
2. The energy storage device according to claim 1, wherein a
thickness of the intermediate layer is not less than 0.1 .mu.m and
not more than 2 .mu.m.
3. The energy storage device according to claim 1, wherein the
electrode is a positive electrode.
4. The energy storage device according to claim 3, wherein the
active material particles of the active material layer contain a
lithium metal composite oxide represented by a chemical composition
of Li.sub.xNi.sub.yMn.sub.zCo.sub.(1-y-z)O.sub.2 (where
0<x.ltoreq.1.3, 0<y<1 and 0<z<1).
5. The energy storage device according to claim 1, wherein the
active material particles include secondary particles, and the
secondary particles have irregularities on surfaces thereof.
6. The energy storage device according to claim 1, wherein a
thickness of the intermediate layer is smaller than an average
particle size D50 of primary particles in the active material
particles.
7. The energy storage device according to claim 1, wherein the
intermediate layer further contains a conductive aid.
8. The energy storage device according to claim 7, wherein the
conductive aid is carbon black.
9. The energy storage device according to claim 1, wherein the
binder of the intermediate layer is a compound having a chitosan
molecular structure.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of Japanese patent
application No. 2015-064186, filed on Mar. 26, 2015, which is
incorporated by reference.
FIELD
[0002] The present invention relates to an energy storage device
such as a nonaqueous electrolyte secondary battery.
BACKGROUND
[0003] A lithium ion secondary battery which includes a positive
electrode including a positive electrode substrate and a positive
active material layer deposited on the positive electrode substrate
has been heretofore known as a nonaqueous electrolyte secondary
battery (e.g. JP-A-2013-065468). In the battery described in
JP-A-2013-065468, the positive active material layer contains a
first active material and a second active material, the first
active material contains secondary particles having a compressive
strength of 85 MPa or more, and the second active material contains
secondary particles having an average particle size smaller than
that of the secondary particles of the first active material. The
positive active material layer is compressed to allow the particles
of the second active material, which have a small particle size, to
enter between the particles of the first active material, so that
the positive active material layer has a relatively large active
material density of 3.65 g/cm.sup.3 or more. Therefore, the battery
described in JP-A-2013-065468 has a relatively high energy
density.
[0004] In the battery described in JP-A-2013-065468, however, the
particles of the active material repeatedly expand and contract as
a charge-discharge reaction is repeated. When the particles of the
active material repeatedly expand and contract, active material
particles abutted against the positive electrode substrate move
away from the positive electrode substrate, so that the positive
active material layer is partially separated from the positive
electrode substrate. Therefore, in the battery described in
JP-A-2013-065468, the internal resistance may be increased by
charge-discharge cycles.
SUMMARY
[0005] The following presents a simplified summary of the invention
disclosed herein in order to provide a basic understanding of some
aspects of the invention. This summary is not an extensive overview
of the invention. It is intended to neither identify key or
critical elements of the invention nor delineate the scope of the
invention. Its sole purpose is to present some concepts of the
invention in a simplified form as a prelude to the more detailed
description that is presented later.
[0006] An object of the present invention is to provide an energy
storage device in which the internal resistance can be inhibited
from being increased by charge-discharge cycles.
[0007] An energy storage device according to an aspect of the
present invention includes an electrode having an electrode
substrate; an active material layer which is disposed to cover a
surface of the electrode substrate and which contains active
material particles; and an intermediate layer which is disposed
between the electrode substrate and the active material layer and
which contains a binder, wherein the active material particles of
the active material layer enter the intermediate layer, and are in
contact with the electrode substrate and the intermediate
layer.
BRIEF DESCRIPTION OF DRAWINGS
[0008] The foregoing and other features of the present invention
will become apparent from the following description and drawings of
an illustrative embodiment of the invention in which:
[0009] FIG. 1 shows an enlarged view of a part of an electrode
assembly of an energy storage device according to an
embodiment.
[0010] FIG. 2 shows a sectional view of a positive electrode, a
negative electrode and a separator which are joined together (II-II
cross-section in FIG. 1).
[0011] FIG. 3 shows a sectional view of a positive electrode in the
energy storage device according to the embodiment.
[0012] FIG. 4 shows a perspective view of the energy storage device
according to the embodiment.
[0013] FIG. 5 shows a front view of the energy storage device
according to the embodiment.
[0014] FIG. 6 shows a sectional view of the energy storage device
along a VI-VI line position in FIG. 4.
[0015] FIG. 7 shows a sectional view of the energy storage device
along a VII-VII line position in FIG. 4.
[0016] FIG. 8 shows a perspective view showing a state in which the
energy storage device according to the embodiment is partially
assembled, where an electrolyte solution filling plug, an electrode
assembly, a current collector and an external terminal are mounted
on a lid plate.
[0017] FIG. 9 shows a view explaining a configuration of the
electrode assembly of the energy storage device according to the
embodiment.
[0018] FIG. 10 shows a perspective view of an energy storage
apparatus of the energy storage device according to the
embodiment.
[0019] FIG. 11 shows a flowchart showing steps in a method for
producing an energy storage device.
[0020] FIG. 12 shows electron microscope photographs of a
cross-section of a positive electrode in Example 1.
[0021] FIG. 13 shows electron microscope photographs of a
cross-section of a positive electrode in Example 2.
[0022] FIG. 14 shows electron microscope photographs of a
cross-section of a positive electrode in Example 3.
[0023] FIG. 15 shows electron microscope photographs of a
cross-section of a positive electrode in Example 4.
[0024] FIG. 16 shows electron microscope photographs of a
cross-section of a positive electrode in Comparative Example 2.
[0025] FIG. 17 shows a graph showing a resistivity ratio with
respect to the number of cycles in each battery.
DESCRIPTION OF EMBODIMENTS
[0026] According to an aspect of the present invention, there is
provided an energy storage device including an electrode having an
electrode substrate; an active material layer which is disposed to
cover a surface of the electrode substrate and which contains
active material particles; and an intermediate layer which is
disposed between the electrode substrate and the active material
layer and which contains a binder, wherein the active material
particles of the active material layer enter the intermediate
layer, and are in contact with the electrode substrate and the
intermediate layer.
[0027] In the energy storage device having the above-mentioned
configuration, the active material particles enter the intermediate
layer, and are in contact with the electrode substrate and the
intermediate layer. When a charge-discharge reaction is repeated in
the energy storage device, the active material particles repeatedly
expand and contract. However, because the active material particles
enter the intermediate layer, and are in contact with the electrode
substrate and the intermediate layer, they are hard to move away
from the electrode substrate and the intermediate layer even when
expanding and contracting. Accordingly, the active material layer
is hard to be separated from the intermediate layer. The active
material particles enter the intermediate layer containing a
binder, and are in contact with the electrode substrate. The binder
of the intermediate layer exists on the circumference of a part
where the active material particles are in contact with the
electrode substrate. Because the binder exists, the active material
particles are hard to move away from the electrode substrate and
the intermediate layer, so that the active material layer is hard
to be separated from the intermediate layer. Accordingly, in the
energy storage device, the internal resistance can be inhibited
from being increased by charge-discharge cycles.
[0028] In the energy storage device, the thickness of the
intermediate layer may be not less than 0.1 .mu.m and not more than
2 .mu.m. The electrode may be a positive electrode.
[0029] In the energy storage device, for example, the active
material particles of the active material layer may contain a
lithium metal composite oxide represented by the chemical
composition of Li.sub.xNi.sub.yMn.sub.zCo.sub.(1-y-z)O.sub.2 (where
0<x.ltoreq.1.3, 0<y<1 and 0<z<1). Accordingly, in
the energy storage device, the active material particles can more
reliably pass through the intermediate layer during production. For
the same reason as described above, separability of the active
material layer from the electrode substrate can be reduced.
[0030] In the energy storage device, the active material particles
may include secondary particles, and the secondary particles may
have irregularities on the surfaces thereof. The convex parts
easily pass through the intermediate layer because they are
projective. Therefore, secondary particles disposed with the convex
parts projecting toward the electrode substrate can more reliably
pass through the intermediate layer during production.
[0031] In the energy storage device, the thickness of the
intermediate layer may be smaller than the average particle size
D50 of the primary particles in the active material particles. With
the above-mentioned configuration, the active material particles
can more reliably pass through the intermediate layer during
production.
[0032] In the energy storage device, the intermediate layer may
contain a conductive aid. The conductive aid may be carbon black.
When the conductive aid is carbon black, more uniform conductivity
can be imparted to the intermediate layer.
[0033] In the energy storage device, the binder of the intermediate
layer may be a compound having a chitosan molecular structure. When
the binder is a compound having a chitosan molecular structure,
adhesion between the intermediate layer and the electrode substrate
can be more reliably retained.
[0034] According to the present invention, the internal resistance
of the energy storage device can be inhibited from being increased
by charge-discharge cycles.
[0035] Hereinafter, one embodiment of the energy storage device
according to the present invention will be described with reference
to FIGS. 1 to 9. Energy storage devices include secondary
batteries, capacitors and so on. In this embodiment, a secondary
battery capable of being charged and discharged will be described
as one example of the energy storage device. The names of
constituent members (constituent elements) in this embodiment are
specific to this embodiment, and may be different from the names of
constituent members (constituent elements) in the background
art.
[0036] An energy storage device 1 according to this embodiment is a
nonaqueous electrolyte secondary battery. More specifically, the
energy storage device 1 is a lithium ion secondary battery which
makes use of electron transfer that occurs with migration of
lithium ions. The energy storage device 1 of this type supplies
electric energy. One or more energy storage devices 1 are used.
Specifically, one energy storage device 1 is used when a required
power and a required voltage are small. On the other hand, when at
least one of a required power and a required voltage is large, the
energy storage device 1 is combined with other energy storage
devices 1, and used in an energy storage apparatus 100. In the
energy storage apparatus 100, energy storage devices 1 used in the
energy storage apparatus 100 supply electric energy.
[0037] The energy storage device 1 has a positive electrode 11 and
a negative electrode 12 as electrodes. Specifically, as shown in
FIGS. 1 to 9, the energy storage device 1 includes an electrode
assembly 2 including the positive electrode 11, the negative
electrode 12 and a separator 4; a case 3 which stores the electrode
assembly 2; and an external terminal 7 disposed on the outside of
the case 3, the external terminal 7 communicating with the
electrode assembly 2. The energy storage device 1 includes, in
addition to the electrode assembly 2, the case 3 and the external
terminal 7, a current collector 5 which makes the electrode
assembly 2 conductive with the external terminal 7; and the
like.
[0038] The electrode assembly 2 is formed by winding a layered
product 22 in which the positive electrode 11 and the negative
electrode 12 are laminated while being insulated from each other by
the separator 4. Accordingly, in the electrode assembly 2, the
separator 4 is disposed between the positive electrode 11 and the
negative electrode 12 facing the positive electrode 11.
[0039] The positive electrode 11 includes a metal foil 111 as a
positive electrode substrate; an intermediate layer 113 which is
formed so as to overlap the metal foil 111 and which contains a
conductive aid and a binder; and a positive active material layer
112 which is formed so as to overlap the intermediate layer 113 and
which contains active material particles. The intermediate layer
113 is not required to contain a conductive aid, but as one
example, an intermediate layer containing a conductive aid will be
described in this embodiment.
[0040] The metal foil 111 is in the form of a belt. The thickness
of the metal foil 111 is normally not less than 10 .mu.m and not
more than 20 .mu.m. The metal foil 111 of the positive electrode in
this embodiment is, for example, an aluminum foil. The positive
electrode 11 has an exposed part 105, which is not covered with the
positive active material layer 112 (a part where the positive
active material layer 112 is not formed), at one end edge of the
belt shape in the width direction, i.e. the short direction.
[0041] The positive active material layer 112 is disposed so as to
face the negative electrode 12. The positive active material layer
112 contains a positive active material and a binder. Specifically,
the positive active material layer 112 contains active material
particles of the positive electrode in an amount of not less than
80% by mass and not more than 98% by mass, a binder in an amount of
not less than 1% by mass and not more than 10% by mass, and a
conductive aid in an amount of not less than 1% by mass and not
more than 10% by mass.
[0042] The active material particles of the positive electrode 11
are particles containing a positive active material capable of
storing and releasing lithium ions. The active material particles
contain 95% by mass or more of the positive active material. The
positive active material is, for example, a lithium metal oxide.
Specifically, the positive active material is, for example, a
composite oxide represented by Li.sub.xMeO.sub.p (Me represents one
or more transition metals) (Li.sub.xCoO.sub.2, Li.sub.xNiO.sub.2,
Li.sub.xMnO.sub.4, Li.sub.xNi.sub.yMn.sub.zCo.sub.(1-y-z)O.sub.2 or
the like), or a polyanion compound represented by
Li.sub.aMe.sub.b(XO.sub.c).sub.d (Me represents one or more
transition metals, and X represents, for example, P, Si, B or V)
(Li.sub.aFe.sub.bPO.sub.4, Li.sub.aMn.sub.bPO.sub.4,
Li.sub.aMn.sub.bSiO.sub.4, Li.sub.aCo.sub.bPO.sub.4F or the like).
The positive active material in this embodiment is a lithium metal
composite oxide represented by the chemical composition of
Li.sub.xNi.sub.yMn.sub.zCo.sub.(1-y-z)O.sub.2 (where
0<x.ltoreq.1.3, 0<y<1 and 0<z<1), specifically
LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2. The average particle size
D50 of the active material particles of the positive electrode 11
is normally not less than 3 .mu.m and not more than 20 .mu.m.
[0043] The positive active material is harder than the metal foil
111. The active material particles of the positive electrode which
contain the positive active material are harder than the metal foil
111. The hardness of each of the positive active material and the
metal foil 111 is determined by measurement using, for example, a
dynamic ultramicro hardness meter manufactured by Shimadzu
Corporation.
[0044] In the positive active material layer 112, active material
particles exist in the form of secondary particles in which primary
particles containing the positive active material are aggregated,
and in the form of primary particles which are not aggregated. That
is, in the positive active material layer 112, both independently
existing primary particles and secondary particles in which primary
particles are aggregated exist. Most of the active material
particles contained in the positive active material layer 112 are
secondary particles in which primary particles are aggregated.
[0045] The active material particles of the positive electrode 11
enter the intermediate layer 113, and are in contact with the metal
foil 111 and the intermediate layer 113. Specifically, the active
material particles of the positive electrode 11 pass through the
intermediate layer 113, and get stuck in the metal foil 111.
[0046] The active material particles of the positive electrode 11
include secondary particles, and the secondary particles have
irregularities on the surfaces thereof. The secondary particle
having irregularities on the surface thereof includes, for example,
a central part with primary particles aggregated in a spherical
form, and a convex part with primary particles projecting outward
from the surface of the central part. For example, the convex part
passes through the intermediate layer 113, and is stuck in the
metal foil 111.
[0047] Examples of the binder to be used in the positive active
material layer 112 include polyvinylidene fluoride (PVdF), a
copolymer of ethylene and vinyl alcohol, polymethyl methacrylate,
polyethylene oxide, polypropylene oxide, polyvinyl alcohol,
polyacrylic acid, polymethacrylic acid and styrene butadiene rubber
(SBR). The binder in this embodiment is polyvinylidene
fluoride.
[0048] The positive active material layer 112 may further contain a
conductive aid such as ketjen black (registered trademark),
acetylene black or graphite. The positive active material layer 112
in this embodiment contains acetylene black as a conductive
aid.
[0049] The intermediate layer 113 is disposed between the metal
foil 111 and the positive active material layer 112. The
intermediate layer 113 does not contain active material particles.
The ratio of the thickness of the intermediate layer 113 to the
thickness of the metal foil 111 of the positive electrode 11 is not
less than 0.005 and not more than 0.2. The thickness of the
intermediate layer 113 is normally not less than 0.1 .mu.m and not
more than 2 .mu.m. The thickness of the intermediate layer 113 is a
thickness at a part where active material particles are not stuck
in the intermediate layer 113. Specifically, the thickness of the
intermediate layer 113 is, for example, a thickness of the
intermediate layer 113 on the circumference of active material
particles which pass through the intermediate layer 113, and get
stuck in the metal foil 111.
[0050] The intermediate layer 113 can be partially formed between
the positive electrode substrate (metal foil) 111 and the positive
active material layer 112. That is, the intermediate layer 113 can
partially cover a surface of the positive electrode substrate
(metal foil) 111.
[0051] The thickness of the intermediate layer 113 is smaller than
the average particle size D50 of primary particles in the active
material particles. The average particle size D50 of the primary
particles is determined from the average particle size of the
primary particles in a scanning electron microscope photograph of a
cross-section of the electrode (positive electrode 11) in the
thickness direction. Specifically, in the photograph of the
cross-section, at least 100 primary particles of active material
particles disposed so as to extend along the metal foil 111 are
randomly selected, the longest diameters of the primary particles
are measured, and the measured values are averaged to determine the
average particle size D50 of the primary particles.
[0052] The ratio of the thickness of the intermediate layer 113 to
the average particle size D50 of the primary particles of the
active material particles is normally not less than 0.05 and not
more than 1.0.
[0053] The intermediate layer 113 can be formed by performing
application in such a manner that the solid content is 2 g/m.sup.2
or less, preferably not less than 0.1 g/m.sup.2 and not more than 1
g/m.sup.2. The solid content is the content of components excluding
components that are volatilized after application during
production, i.e. a basis weight.
[0054] The intermediate layer 113 contains a binder at a mass ratio
of not less than 0.5 and not more than 5 relative to a conductive
aid. The intermediate layer 113 contains the conductive aid in an
amount of not less than 30% by mass and not more than 80% by mass.
The intermediate layer 113 contains the binder in an amount of not
less than 20% by mass and not more than 70% by mass.
[0055] The intermediate layer 113 is softer than the active
material, and softer than the metal foil 111. The hardness of each
thereof is determined by, for example, a Vickers hardness test. The
hardness of the intermediate layer 113 can be adjusted by, for
example, changing the amount ratio between the binder and the
conductive aid.
[0056] The conductive aid is at least one selected from the group
consisting of carbon black and graphite. Examples of the carbon
black include ketjen black (registered trademark) and acetylene
black. In this embodiment, the intermediate layer 113 contains
carbon black as a conductive aid.
[0057] The binder is at least one selected from the group
consisting of a compound having a chitosan molecular structure,
polyvinylidene fluoride (PVdF), a copolymer of ethylene and vinyl
alcohol, polymethyl methacrylate, polyethylene oxide, polypropylene
oxide, polyvinyl alcohol, polyacrylic acid, polymethacrylic acid
and styrene butadiene rubber (SBR). In this embodiment, the
intermediate layer 113 contains at least a compound having a
chitosan molecular structure as a binder.
[0058] Examples of the compound having a chitosan molecular
structure include crosslinked polymers of cellulose and a chitosan
pyrrolidone carboxylic acid salt, and derivatives of chitin or
chitosan.
[0059] The negative electrode 12 includes a metal foil 121 as a
negative electrode substrate, and a negative active material layer
122 formed on the metal foil 121. The metal foil 121 is in the form
of a belt. The metal foil 121 of the negative electrode in this
embodiment is, for example, a copper foil. The negative electrode
12 has an exposed part 105, which is not covered with the negative
active material layer 122 (a part where the negative active
material layer is not formed), at the other end edge of the belt
shape in the width direction, i.e. the short direction (on a side
opposite to the exposed part 105 of the positive electrode 11).
[0060] The negative active material layer 122 contains a negative
active material and a binder.
[0061] The negative active material is, for example, carbon
materials such as graphite, hardly graphitizable carbon and easily
graphitizable carbon, or materials which undergo an alloying
reaction with lithium ions, such as silicon (Si) and tin (Sn). The
negative active material in this embodiment is graphite.
[0062] The binder to be used in the negative active material layer
122 is the same as the binder used in the positive active material
layer 112. The binder in this embodiment is polyvinylidene
fluoride.
[0063] The negative active material layer 122 may further contain a
conductive aid such as ketjen black (registered trademark),
acetylene black or graphite. The negative active material layer 122
in this embodiment does not contain a conductive aid.
[0064] The separator 4 is a member having insulation quality. The
separator 4 is in the form of a belt. The separator 4 is disposed
between the positive electrode 11 and the negative electrode 12.
Accordingly, in the electrode assembly 2 (specifically layered
product 22), the positive electrode 11 and the negative electrode
12 are insulated from each other. The separator 4 holds an
electrolyte solution in the case 3. Accordingly, during
charge-discharge of the energy storage device 1, lithium ions move
between the positive electrode 11 and the negative electrode 12
which are alternately laminated with the separator 4 sandwiched
therebetween.
[0065] The separator 4 is formed as a porous material using, for
example, a woven fabric, a nonwoven fabric or a porous membrane.
The material of the separator 4 is a polymer compound, glass,
ceramic or the like. Examples of the polymer compound include
polyacrylonitrile (PAN), polyamide (PA), polyesters such as
polyethylene terephthalate (PET), polyolefins (PO) such as
polypropylene (PP) and polyethylene (PE), and cellulose.
[0066] The width of the separator 4 (dimension of the belt shape in
the short direction) is slightly larger than the width of the
negative active material layer 122. The separator 4 is disposed
between the positive electrode 11 and the negative electrode 12
which are joined together while being positionally shifted in the
width direction so that the positive active material layer 112 and
the negative active material 122 overlap each other.
[0067] In the electrode assembly 2 in this embodiment, the positive
electrode 11 and the negative electrode 12 which are configured as
described above are wound while being insulated from each other by
the separator 4. That is, in the electrode assembly 2 in this
embodiment, the layered product 22 of the positive electrode 11,
the negative electrode 12 and the separator 4 is wound.
[0068] As shown in FIG. 9, the exposed part 105 of the positive
electrode 11 and the exposed part 105 of the negative electrode 12
do not overlap each other with the positive electrode 11 and the
negative electrode 12 laminated together. That is, the exposed part
105 of the positive electrode 11 projects in the width direction
from a region where the positive electrode 11 and the negative
electrode 12 overlap each other, and the exposed part 105 of the
negative electrode 12 projects in the width direction (direction
opposite to the direction in which the exposed part 105 of the
positive electrode 11 projects) from the region where the positive
electrode 11 and the negative electrode 12 overlap each other. The
electrode assembly 2 is formed by winding the positive electrode
11, the negative electrode 12 and the separator 4 in a laminated
state, i.e. the layered product 22. A part where only the exposed
part 105 of the positive electrode 11 or the exposed part 105 of
the negative electrode 12 is laminated forms an exposed layered
part 26 in the electrode assembly 2.
[0069] The exposed layered part 26 is a part which is made
conductive with the current collector 5 in the electrode assembly
2. The exposed layered part 26 is sectioned into two parts
(bisected exposed layered parts) 261 with a hollow part 27
sandwiched therebetween when viewed in the winding center direction
of the wound positive electrode 11, negative electrode 12 and
separator 4.
[0070] The exposed layered part 26 configured as described above is
provided in each of the electrodes of the electrode assembly 2.
That is, the exposed layered part 26 where only the exposed part
105 of the positive electrode 11 is laminated forms the exposed
layered part of the positive electrode 11 in the electrode assembly
2, and the exposed layered part 26 where only the exposed part 105
of the negative electrode 12 is laminated forms the exposed layered
part of the negative electrode 12 in the electrode assembly 2.
[0071] The case 3 includes a case main body 31 having an opening,
and a lid plate 32 which covers (closes) the opening of the case
main body 31. The case 3 stores in the internal space an
electrolyte solution together with the electrode assembly 2, the
current collector 5 and so on. The case 3 is formed of a metal
resistant to the electrolyte solution. The case 3 is formed of, for
example, an aluminum-based metal material such as aluminum or an
aluminum alloy. The case 3 may be formed of a metal material such
as stainless steel or nickel, a composite material with a resin of
nylon etc. bonded to aluminum, or the like.
[0072] The electrolyte solution is a nonaqueous solution-based
electrolyte solution. The electrolyte solution is obtained by
dissolving an electrolyte salt in an organic solvent. Examples of
the organic solvent include cyclic carbonic acid esters such as
propylene carbonate and ethylene carbonate, and chain carbonates
such as dimethyl carbonate, diethyl carbonate and ethylmethyl
carbonate. Examples of the electrolyte salt include LiClO.sub.4,
LiBF.sub.4 and LiPF.sub.6. The electrolyte solution is one obtained
by dissolving 1 mol/L of LiPF.sub.6 in a mixed solvent prepared by
mixing propylene carbonate, dimethyl carbonate and ethylmethyl
carbonate at a ratio of propylene carbonate:dimethyl
carbonate:ethylmethyl carbonate=3:2:5.
[0073] The case 3 is formed by bonding the circumferential edge
part of the opening of the case main body 31 and the
circumferential edge part of the rectangular lid plate 32 to each
other with one superimposed on the other. The case 3 has an
internal space defined by the case main body 31 and the lid plate
32. In this embodiment, the circumferential edge part of the
opening of the case main body 31 and the circumferential edge part
of the lid plate 32 are bonded to each other by welding.
[0074] Hereinafter, the long-side direction of the lid plate 32 is
an X-axis direction, the short-side direction of the lid plate 32
is a Y-axis direction, and the normal direction of the lid plate 32
is a Z-axis direction as shown in FIG. 4.
[0075] The case main body 31 has a prismatic cylindrical shape in
which one end in the opening direction (Z-axis direction) is
covered (i.e. bottomed prismatic shape).
[0076] The lid plate 32 is a plate-shaped member which covers the
opening of the case main body 31. Specifically, the lid plate 32 is
in contact with the case main body 31 so as to cover the opening of
the case main body 31. More specifically, the circumferential edge
part of the lid plate 32 is superimposed on the circumferential
edge of the opening of the case main body 31 in such a manner that
the lid plate 32 covers the opening. The boundary part between the
lid plate 32 and the case main body 31 is welded with the lid plate
32 superimposed on the circumferential part of the opening. In this
way, the case 3 is formed.
[0077] The lid plate 32 has a contour shape corresponding to the
circumferential part of the opening of the case main body 31 when
viewed in the Z-axis direction. That is, the lid plate 32 is a
rectangular plate material which is long in the X-axis direction
when viewed in the Z-axis direction. The four corners of the lid
plate 32 are in the form of a circular arc.
[0078] The lid plate 32 includes a gas release valve 321 capable of
discharging a gas in the case 3 to the outside. The gas release
valve 321 discharges a gas from the inside of the case 3 to the
outside when the internal pressure of the case 3 increases to a
preset pressure. The gas release valve 321 is provided at the
central part of the lid plate 32 in the X-axis direction.
[0079] The case 3 is provided with an electrolyte solution filling
hole for injecting an electrolyte solution. The electrolyte
solution filling hole establishes communication between the inside
and the outside of the case 3. The electrolyte solution filling
hole is provided in the lid plate 32.
[0080] The electrolyte solution filling hole is sealed (covered)
with an electrolyte solution filling plug 326. The electrolyte
solution filling plug 326 is fixed to the case 3 (lid plate 32 in
the example in this embodiment) by welding.
[0081] The external terminal 7 is a part which is electrically
connected to the external terminal 7 of another energy storage
device 1, external equipment or the like. The external terminal 7
is formed of a member having conductivity. For example, the
external terminal 7 is formed of a metal material having high
weldability, such as an aluminum-based metal material such as
aluminum or an aluminum alloy, or a copper-based metal material
such as copper or a copper alloy.
[0082] The external terminal 7 has a surface 71 to which a bus bar
etc. can be welded. The surface 71 is a flat surface. The external
terminal 7 is in the form of a plate extending along the lid plate
32. Specifically, the external terminal 7 is in the form of a
rectangular plate when viewed in the Z-axis direction.
[0083] The current collector 5 is disposed in the case 3, and
directly or indirectly connected to the electrode assembly 2 in
such a manner that a current can be passed between the current
collector 5 and the electrode assembly 2. The current collector 5
in this embodiment is connected to the electrode assembly 2 via a
clip member 50 in such a manner that a current can be passed
between the current collector 5 and the electrode assembly 2. That
is, the energy storage device 1 includes the clip member 50 that
connects the electrode assembly 2 and the current collector 5 to
each other in such a manner that a current can be passed
therebetween.
[0084] The current collector 5 is formed of a member having
conductivity. As shown in FIG. 6, the current collector 5 is
disposed along the inner surface of the case 3.
[0085] The current collector 5 is disposed on each of the positive
electrode 11 and the negative electrode 12 in the energy storage
device 1. In the energy storage device 1 according to this
embodiment, the current collector 5 is disposed on each of the
exposed layered part 26 of the positive electrode 11 and the
exposed layered part 26 of the negative electrode 12 of the
electrode assembly 2 in the case 3.
[0086] The current collector 5 of the positive electrode 11 and the
current collector 5 of the negative electrode 12 are formed of
different materials. Specifically, the current collector 5 of the
positive electrode 11 is formed of, for example, aluminum or an
aluminum alloy, and the current collector 5 of the negative
electrode 12 is formed of, for example, copper or a copper
alloy.
[0087] In the energy storage device 1 according to this embodiment,
the electrode assembly 2 (specifically, electrode assembly 2 and
current collector 5) stored in a bag-shaped insulating cover 6 is
stored in the case 3.
[0088] A method for producing the energy storage device according
to this embodiment will now be described with reference to FIG.
11.
[0089] A composition containing a conductive aid and a binder is
applied to an electrode substrate (step S1). A mixture containing
active material particles is applied to the applied composition
(step S2). The applied mixture is pressed to prepare an electrode
(positive electrode) (step S3). An electrode assembly is formed by
joining a positive electrode, a separator and a negative electrode
together (step S4). The electrode assembly is placed in the case,
and an electrolyte solution is injected into the case to assemble
the energy storage device (step S5).
[0090] In step S1, a composition for an intermediate layer, which
contains a conductive aid, a binder and a solvent is applied to
each of both surfaces of the metal foil 111 for a positive
electrode to form the intermediate layer 113. As an application
method for forming the intermediate layer 113, a common method is
employed.
[0091] In step S2, a mixture containing a positive active material,
a binder and a solvent is applied to the outside surface of each
formed intermediate layer 113 to form the positive active material
layer 112. As an application method for forming the positive active
material layer 112, a common method is employed.
[0092] In step S3, for example, a roll pressing method is employed.
Specifically, the metal foil 111, the intermediate layer 113 and
the positive active material layer 112, which are stacked one on
another, are pressed while being sandwiched between a pair of
rolls. Accordingly, the active material particles of the positive
electrode 11 are made to enter the intermediate layer 113, pass
through the intermediate layer 113 and get stuck in the metal foil
111. The active material particles are then brought into contact
with the metal foil 111. The negative electrode 12 can be similarly
prepared by forming the negative active material layer 122 on the
metal foil 121 for a negative electrode.
[0093] In step S3, the pressing pressure is preferably not less
than 10 kgf/mm and not more than 100 kgf/mm. In pressing, it is
preferable that the roll has a diameter of .PHI.360 mm and a
temperature of 150.degree. C.
[0094] In step S4, the layered product 22 with the separator 4
sandwiched between the positive electrode 11 and the negative
electrode 12 is wound to form the electrode assembly 2. In
formation of the electrode assembly 2, the layered product 22 is
prepared by joining the positive electrode 11, the separator 4 and
the negative electrode 12 in such a manner that the positive active
material layer 112 and the negative active material layer 122 face
each other with the separator 4 interposed therebetween. Next, the
layered product 22 is wound to form the electrode assembly 2.
[0095] In step S5, the electrode assembly 2 is placed in the case
main body 31 of the case 3, the opening of the case main body 31 is
covered with the lid plate 32, and the electrolyte solution is
injected into the case 3. At the time of covering the opening of
the case main body 31 with the lid plate 32, the electrode assembly
2 is placed in the case main body 31, and the opening of the case
main body 31 is covered with the lid plate 32 with the positive
electrode 11 made conductive with one external terminal 7 and the
negative electrode 12 made conductive with the other external
terminal 7. At the time of injecting the electrolyte solution into
the case 3, the electrolyte solution is injected into the case 3
through an injection hole of the lid plate 32 of the case 3.
[0096] In the energy storage device 1 of this embodiment which is
configured as described above, the active material particles enter
the intermediate layer 113, and are in contact with the electrode
substrate (metal foil) 111 and the intermediate layer 113 of the
positive electrode 11. When a charge-discharge reaction is repeated
in the energy storage device 1, the active material particles
repeatedly expand and contract. However, because the active
material particles enter the intermediate layer 113, and are in
contact with the electrode substrate (metal foil) 111 and the
intermediate layer 113 of the positive electrode 11, they are hard
to move away from the electrode substrate (metal foil) 111 and the
intermediate layer 113 even when expanding and contracting.
Accordingly, the positive active material layer 112 is hard to be
separated from the intermediate layer 113. The active material
particles enter the intermediate layer 113 containing a binder, and
are in contact with the electrode substrate (metal foil) 111 and
the intermediate layer 113. The binder of the intermediate layer
113 exists on the circumference of a part where the active material
particles are in contact with the electrode substrate (metal foil)
111. Because the binder exists, the active material particles are
hard to move away from the electrode substrate (metal foil) 111 and
the intermediate layer 113, so that the positive active material
layer 112 is hard to be separated from the intermediate layer 113.
Accordingly, in the energy storage device 1, the internal
resistance can be inhibited from being increased by
charge-discharge cycles.
[0097] In the energy storage device 1, the active material
particles of the positive electrode 11 pass through the
intermediate layer 113, and get stuck in the electrode substrate
(metal foil) 111 of the positive electrode 11. Because the active
material particles are stuck in the electrode substrate (metal
foil) 111, they are hard to move away from the electrode substrate
(metal foil) 111 and the intermediate layer 113 even when expanding
and contracting as described above. Accordingly, the positive
active material layer 112 is hard to be separated from the
intermediate layer 113. The active material particles pass through
the intermediate layer 113 containing a binder, and get stuck in
the electrode substrate (metal foil) 111. The binder of the
intermediate layer 113 exists on the circumference of a part where
the active material particles are stuck in the electrode substrate.
Because the binder exists, the active material particles are hard
to move away from the electrode substrate (metal foil) 111 and the
intermediate layer 113, so that the positive active material layer
112 is hard to be separated from the intermediate layer 113.
Accordingly, in the energy storage device 1, the internal
resistance can be inhibited from being increased by
charge-discharge cycles. The active material particles break
through an oxidized surface film existing on a surface of the
electrode substrate (metal foil) 111, and get stuck in and are in
contact with the electrode substrate (metal foil) 111. Accordingly,
the internal resistance can be inhibited from being increased, and
further, performance, such as power, of the energy storage device 1
can be improved.
[0098] In the energy storage device 1, a conductive aid may exist
on the circumference of the part where the active material
particles are stuck in the electrode substrate because the
intermediate layer 113 contains the conductive aid. Because the
conductive aid exists, conductivity between the stuck active
material particles and the electrode substrate (metal foil) 111 is
reliably secured.
[0099] In the energy storage device 1, the active material
particles of the positive active material layer 112 contain a
lithium metal composite oxide represented by the chemical
composition of Li.sub.xNi.sub.yMn.sub.zCo.sub.(1-y-z)O.sub.2 (where
0<x.ltoreq.1.3, 0<y<1 and 0<z<1). Accordingly, the
active material particles can be more reliably stuck in the metal
foil 111 during production.
[0100] In the energy storage device 1, the active material
particles of the positive electrode 11 include secondary particles,
and the secondary particles have irregularities on the surfaces
thereof. The convex parts easily get stuck in the metal foil
because they are projective. Therefore, secondary particles
disposed with the convex parts projecting toward the metal foil 111
can more reliably get stuck in the metal foil 111 during
production.
[0101] In the energy storage device 1, the thickness of the
intermediate layer 113 is smaller than the average particle size
D50 of the primary particles in the active material particles. With
the above-mentioned configuration, the active material particles
can more reliably pass through the intermediate layer 113 during
production.
[0102] In energy storage device 1, the conductive aid of the
intermediate layer is carbon black. Accordingly, more uniform
conductivity can be imparted to the intermediate layer. The binder
of the intermediate layer is a compound having a chitosan molecular
structure. Accordingly, adhesion between the intermediate layer and
the electrode substrate can be more reliably retained.
[0103] The energy storage device according to the present invention
is not limited to the above-described embodiment, and it is
needless to say that various changes can be made in the range
without departing from the spirit of the present invention. For
example, to the configuration of an embodiment can be added the
configuration of another embodiment, or a part of the configuration
of an embodiment can be replaced by the configuration of another
embodiment. Further, a part of the configuration of an embodiment
can be eliminated.
[0104] In the above-described embodiment, the energy storage device
1 including the positive electrode 11 having the intermediate layer
113 has been described in detail, but in the present invention, the
negative electrode 12 may have an intermediate layer, and the
active material particles of the negative electrode 12 may be stuck
in the metal foil 121 of the negative electrode 12.
[0105] In the above-described embodiment, an electrode with an
active material layer disposed on each of both surfaces of a metal
foil of each electrode has been described, but in the energy
storage device according to the present invention, the positive
electrode 11 or the negative electrode 12 may include an active
material layer on only one surface of the metal foil.
[0106] In the above-described embodiment, the energy storage device
1 including the electrode assembly 2 formed by winding the layered
product 22 has been described in detail, but the energy storage
device according to the present invention may include the layered
product 22 which is not wound. Specifically, the energy storage
device may include an electrode assembly in which a positive
electrode, a separator, a negative electrode and a separator each
formed into a rectangular shape are stacked in this order over a
plurality of times.
[0107] In the above-described embodiment, the energy storage device
1 is used as a nonaqueous electrolyte secondary battery (e.g.
lithium ion secondary battery) capable of being charged and
discharged, but any of the type and volume (capacity) can be
applied to the energy storage device 1. In the above-described
embodiment, a lithium ion secondary battery has been described as
one example of the energy storage device 1, but the energy storage
device is not limited thereto. For example, the present invention
is applicable to various secondary batteries, and energy storage
devices of capacitors such as electric double layer capacitors.
[0108] The energy storage device 1 (e.g. battery) may be used in
the energy storage apparatus 100 shown in FIG. 10 (battery module
when the energy storage device is a battery). The energy storage
apparatus 100 includes at least two energy storage devices 1, and a
bus bar member 91 which electrically connects two (different)
energy storage devices 1. Here, the technique according to the
present invention may be applied to at least one energy storage
device.
EXAMPLES
[0109] A nonaqueous electrolyte secondary battery (lithium ion
secondary battery) was produced in the manner shown below.
Example 1
(1) Preparation of Positive Electrode
[0110] A composition for an intermediate layer was prepared by
mixing a conductive aid (carbonaceous material; acetylene black;
average particle size: 35 nm), a binder (chitosan; product
designation "DCN"; manufactured by Dainichiseika Color &
Chemicals Mfg. Co., Ltd.) and n-methyl-2-pyrrolidone (NMP) at a
mass ratio of conductive aid/binder/NMP=5/5/90. The prepared
composition was applied onto each of both surfaces of a 15
.mu.m-thick aluminum foil in such a manner that the amount of the
composition after drying was 0.2 g/m.sup.2. NMP as a solvent, a
conductive aid (acetylene black), a binder (PVdF) and a positive
active material (LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2) were
mixed and kneaded to prepare a mixture for a positive electrode.
The blending amounts of the conductive aid, the binder and the
positive active material were 4.5% by mass, 4.5% by mass and 91% by
mass, respectively. The prepared mixture for a positive electrode
was applied onto the composition for an intermediate layer in such
a manner that the amount of the mixture after drying was 10
mg/cm.sup.2. After drying, roll pressing was performed so that the
active material filling density in the positive active material
layer was 3 g/mL. The pressing condition (line pressure) during
roll pressing was 30 kgf/mm. Thereafter, vacuum drying was
performed to remove moisture.
(2) Preparation of Negative Electrode
[0111] Graphite having an average particle size D50 of 10 .mu.m was
used as a negative active material. PVdF was used as a binder. A
mixture for a negative electrode was prepared by mixing and
kneading NMP as a solvent, a binder and a negative active material.
The binder was blended in an amount of 7% by mass, and the negative
active material was blended in an amount of 93% by mass. The
prepared mixture for a negative electrode was applied onto a 10
.mu.m-thick copper foil in such a manner that the amount of the
mixture after drying was 5 mg/cm.sup.2. After drying, roll pressing
was performed so that the active material filling density in the
negative electrode mixture was 1.5 g/mL, and vacuum drying was
performed to remove moisture.
(3) Separator
[0112] A polyethylene microporous membrane having a width of 10 cm
and a thickness of 21 .mu.m was provided as a separator.
(4) Preparation of Electrolyte Solution
[0113] As an electrolyte solution, one prepared by the following
method was used. A solvent obtained by mixing propylene carbonate,
dimethyl carbonate and ethylmethyl carbonate in an amount of 30% by
volume, 40% by volume and 30% by volume, respectively, was provided
as a nonaqueous solvent, and LiPF.sub.6 was dissolved in this
nonaqueous solvent in such a manner that the salt concentration was
1.2 mol/L, thereby preparing an electrolyte solution.
(5) Placement of Electrode Assembly in Case
[0114] Using the positive electrode, the negative electrode, the
electrolyte solution, the separator and a case, a battery was
produced by a common method.
[0115] First, a sheet-shaped product formed by laminating the
positive electrode and the negative electrode with the separator
disposed therebetween was wound. Next, an electrode assembly formed
by winding the sheet-shaped product was placed in a case main body
of an aluminum prismatic container as a case. Subsequently, the
positive electrode and the negative electrode were electrically
connected to two external terminals, respectively. Further, a lid
plate was attached to the case main body. The electrolyte solution
was injected into the case through an electrolyte solution filling
port formed in the lid plate of the case. Finally, the electrolyte
solution filling port of the case was sealed to seal the case.
Example 2
[0116] Except that the composition for an intermediate layer was
applied in such a manner that the amount of the composition was 0.3
g/m.sup.2, the same procedure as in Example 1 was carried out to
produce a lithium ion secondary battery.
Example 3
[0117] Except that the composition for an intermediate layer was
applied in such a manner that the amount of the composition was 0.4
g/m.sup.2, the same procedure as in Example 1 was carried out to
produce a lithium ion secondary battery.
Example 4
[0118] Except that the composition for an intermediate layer was
applied in such a manner that the amount of the composition was 0.6
g/m.sup.2, the same procedure as in Example 1 was carried out to
produce a lithium ion secondary battery.
Comparative Example 1
[0119] Except that the composition for an intermediate layer was
applied in such a manner that the amount of the composition was 0.2
g/m.sup.2, and the line pressure during roll pressing was 5 kgf/mm,
the same procedure as in Example 1 was carried out to produce a
lithium ion secondary battery.
Comparative Example 2
[0120] Except that the intermediate layer was not prepared, the
same procedure as in Example 1 was carried out to produce a lithium
ion secondary battery.
<Observation with Electron Microscope>
[0121] At the time when the positive electrode was prepared, the
positive electrode was cut along the thickness direction, and the
cross-section was observed with a scanning electron microscope.
Examples of observation images in the examples and the comparative
examples are shown in FIGS. 12 to 16. The straight line segment
shown under each observation image shows a scale.
[0122] In the positive electrodes in Examples 1 to 4, the active
material particles passed through the intermediate layer, and got
stuck in (intruded into) the metal foil. In the positive electrode
in Example 4, the active material particles passed through the
intermediate layer, and were in contact with the metal foil and the
intermediate layer. On the other hand, in the positive electrode in
Comparative Example 1, the active material particles entered the
intermediate layer, but were not in contact with the metal foil. In
the positive electrode in Comparative Example 2, the active
material particles were merely stuck in the metal foil.
<Evaluation of Battery Performance (Internal Resistance)>
[0123] The resistance value ratio with respect to the number of
charge-discharge cycles was determined by a common method to
evaluate battery performance (internal resistance). FIG. 17 shows
evaluation results from a graph prepared by plotting measured
values with the number of charge-discharge cycles shown on the
abscissa and the resistance value ratio shown on the ordinate. The
resistance value is represented by a ratio (relative value) when
the resistance value corresponding to 0 cycles in the battery of
Comparative Example 2 is set as 100.
[0124] In the case of Comparative Example 2 in which the
intermediate layer was absent, the resistance value ratio steadily
increased as the number of cycles increased. On the other hand,
when the intermediate layer was present, an increase in resistance
value ratio slowed as the applied amount of the composition for an
intermediate layer increased. This showed that existence of the
intermediate layer and the amount of the composition for an
intermediate layer evidently contributed to an improvement in
battery performance. For the relationship between whether or not
the active material gets stuck in the metal foil and the resistance
value ratio, a comparison was made between Example 1 and
Comparative Example 1. The result showed that when the active
material was stuck in the metal foil (Example 1), the resistance
value ratio was evidently lower as compared to the case where the
roll pressing pressure on the composition for an intermediate layer
was decreased, so that the active material was not stuck in the
metal foil (Comparative Example 1). It has become apparent that the
active material getting stuck in the electrode foil also
contributes to a reduction in resistance value ratio.
* * * * *